Electron densities have been measured in the subsonic portion of a free‐expansion argon jet by microwave and Langmuir‐probe methods. Microwave phase‐shift measurements were made with an interferometer in such a way that the spatial variation of electron density could be determined. The data were analyzed with a ray‐tracing technique.Langmuir‐probe measurements were made with cylindrical and flat probes under essentially collision‐free conditions, and over a range of probe radius to Debye length ratios of about 10–50. Both electron and ion collection were measured and analyzed. Three methods of analyzing the electron‐current part of the characteristic were compared. It was found that one method gave values two or three times higher than another method. The discrepancy was due to the failure of the experimental Langmuir‐probe characteristic to follow the theoretical one in the vicinity of plasma potential: the so‐called ``knee'' of the curve. This difficulty has been overcome by analyzing the data with the aid of theoretical particle‐collection curves.